Beam Pull Out Tests of NSM – FRP and Steel Bars in Concrete

ABSTRACT

This paper describes an experimental investigation regarding a relatively new strengthening method for flexural concrete members, known as NSM technology (near-surface mounting). The study included fourteen simply supported concrete beams, strengthened by placing either steel or carbon FRP bars in grooves cut on the tension face of the member and bonding with epoxy paste; specimens were loaded in bending till the occurrence of failure by debonding. Nine of the specimens were fabricated to have the bar fully bonded in one half-span, whereas the bonded length in the other half span of the typical member was limited. The bonded length was a parameter of study in the investigation. NSM bars were fully bonded in the case of the remaining five specimens. An objective of this experimental research was to examine the bond strength of the NSM method using realistic stress field conditions (beam pullout tests rather than standard pullout) while at the same time obtaining test results for comparison with those of the direct pull out tests. From the experimental evidence it is concluded that flexural curvature, which occurs in the beam-type tests, has a significant effect on the mode of failure of the upgraded system and on the strength of the failure interface, with a magnitude that depends strongly on the mechanical properties and the surface pattern of the bar. With increasing height of the surface deformations of smooth bars, the failure mode gradually changes from pullout at the bar-epoxy interface, to failure at the epoxy-concrete interface which is also observed in standard pullout tests. In this case, bond strength obtained from the beam tests is in general greater than the respective value recorded from direct pull out tests. This increase is attributed to the additional friction generated on the bar and on the epoxy filler’s lateral surface owing tocurvature compatibility between the bar and the surrounding concrete in the bending beam.